Improve rest api paths for likenft queries

[elise-ng]

Ref oursky/likecoin-chain#275

One question, would you prefer:

• /classes/by_iscn/{iscn_id_prefix} and /classes/by_account/{account} or
• /iscn-classes/{iscn_id_prefix} and /account-classes/{account}?

[hochiw]

don't really have a strong preference but I guess /iscn-classes/{iscn_id_prefix} and /account-classes/{account} looks better

[rickmak]

Let do /iscn-classes/{iscn_id_prefix} and /account-classes/{account}

Since we won't have use-case for returning all Classes, making the inheritance appear in the API is not wise.

[elise-ng]

updated to /iscn_classes/{iscn_id_prefix} and /account_classes/{account}

[elise-ng]

on second thought we could also do /iscn/{iscn id}/classes and /account/{addr}/classes nvm, wasn't thinking clearly, we have the list all queries as well and this pattern won't serve that usage

currently for the reverse query it's /classes/{class id}/iscn and classes/{class id}/account